The invention relates to tool holding systems, and, more particularly, to a system for holding electrodes for accurate machining thereof and electric discharge machining therewith.
It is generally understood that to obtain good results in machining operations it is necessary to have both precisely formed cutting tools as well as fixturing for the tools and parts that provides for accurate and predictable relative alignment between the tool and the piece to be machined. This is particularly true in electric discharge machining (EDM). In an EDM process, the cutting tool is brought into close proximity to the metal work surface. The electrode cutting tool serves as the cathode and the metal workpiece is the anode in an EDM operation with voltage developed therebetween creating an electric arc which erodes the metal surface of the workpiece so that it assumes the shape of a mirror image of the proximate surface of the electrode. EDM operations allow for complex shapes such as intricately shaped cavities to be formed in workpieces that otherwise may be difficult to obtain with more conventional metal cutting types of machining operations. As is apparent, the shape and dimensions of the electrode are of paramount importance in terms of the final shape of the workpiece that is being machined with the EDM process.
Accordingly, electrode manufacture must permit for very precise, intricate shapes to be machined onto the electrode blank with a very high accuracy of repetition. Even assuming this occurs, the subsequent set up of the holder and electrode in an EDM machine if not done in a precise and predictable fashion can ruin a machining operation. For instance, it is known to use round shaped electrode posts that have a transverse indexing pin with the post adapted to be received in a round bore of a fixture block. The fixture block has four standing pins that are adjustably spaced ninety from each other about the fixture block bore for abutting the post indexing pin to angularly locate the post in a C direction. The bore includes a hydraulic chuck for clamping onto the post in a direction transverse to its axis to keep it fixed in the X and Y directions.
This system faces several shortcomings. Relying on adjustable standing pins to set the position of the post in the C direction is not very reliable as the pins may be adjusted in a different manner from one worker to the next. Further, it has been found that during machining of the electrode blank such as in milling or grinding operations, the hydraulic clamping provided by the block chuck may not keep the posts sufficiently stationary to provide for precise forming of the electrode.
Another problem with the EDM system described above is in typical post holders the mounting surface at the top of the post or on an enlarged flange thereof is a smooth flat surface with the electrode having a rear flat surface which is then glued to the post mounting surface for attaching the electrode thereto. To provide for extra holding force, a fastener can be threaded into aligned apertures of the post and the electrode through a throughbore countersunk in the post for inserting the fastener through the rear of the post and seating it in the counterbore therein. Where the electrode is adhered to the post, once the electrode is worn so that it is no longer useful for the particular EDM operation for which it is originally designed, both the electrode and the post holder to which it is adhered generally have to be disposed or stored for possible later use such as by modification thereto. This can result in a tremendous waste of electrode post holders as they are not able to be reused with different electrodes. In this regard, it is not unusual to see EDM machine shops stocking hundreds of posts with electrodes adhered thereto. Further, the glue attachment between the mounting surfaces of the post and electrode do not provide for X, Y and C direction locating of the electrode relative to the post.
Because the posts are generally disposed after the electrode is consumed, the fastener is typically advanced into a counterbore of the post for rear mounting of the electrode thereto, as there is no need to be able to access the fastener with the post received and held in its fixture. Accordingly, front mounting of the electrodes to the post such as via a counterbore in the electrode is not typically done.
There are post holders that have been developed which utilize a raised peg for being press fit in a corresponding opening in the electrode. However, this press fit system also relies upon glue to keep the electrode in place during machining operations and thus the post does not provide the advantage of being reusable with different electrodes. Similarly, other mounting systems have been used with cutouts or openings in one of the electrode and post for receiving projecting portions of the other of the electrode and post therein with fasteners for clamping of the electrode to the post also provided. However, each of these systems relies on very precise cutting or drilling operations to form the openings and/or projections in and on the post and electrode that are difficult to achieve to obtain accurate and reproducible mounting of the electrode to the post. Further, these systems do not provide consistent locating of the electrode to the post in the X,Y and C directions, as is desired.
In this regard, machinists have been known to use post holders formed of square stock having raised cross-shaped portions machined on the mounting face thereof for being interfit into a cross-shaped recessed groove machined onto the back of the electrode mounting surface for proper X,Y and C direction locating of the electrode on the post. However, this approach has not seen widespread acceptance due to some significant drawbacks therewith not the least of which is the fact that square tool holder posts are not readily available as standard items for EDM operations similar to round tool holder posts such as those offered by System 3R International AB of Sweden. One reason for this is the common use of v-fixtures for holding of the posts during machining operations for electrodes and workpieces. Where a square post is clamped onto a v-fixture, the exposed sides of the square post can extend generally at a 45xc2x0 angle to the vertical which is undesirable for establishing the dimensions of the carbon electrode relative to the electrode holder as there are no vertical or horizontal flats to be used as reference faces.
Accordingly, there is a need for a tool holding system, and in particular a post holder for electrodes that allows for precise and repeatable positioning of the electrode relative to the posts in the X,Y and C directions, and which allows the posts to be reused with different electrodes with the electrodes being able to be repeatably mounted in known predetermined positions relative to the posts. The posts should allow a machinist to obtain accurate machined dimensions on the electrode when it is mounted thereto. In furtherance of the above, an electrode machining system that provides for precise fixturing of the electrode holder in known predetermined positions relative thereto would likewise be desirable.
In accordance with the present invention, a tool holding system is provided which allows for tools, and in particular electrodes to be removably attached on post holders in precise predetermined positions in terms of the location of the electrode relative to the posts in the X, Y and C directions. As the electrodes are releasably attached to the posts, it is critical for precise machining of the electrodes and for subsequent EDM operations that electrodes be attached to the posts in an accurate and repeatable manner. The present tool holding system utilizes interfitting surface portions which define predetermined positions of the tool on the post relative to the X-Y directions transverse to the post axis and the C-direction angularly about the axis so as to provide the necessary accurate and repeatable mounting of the electrode to the post. Further, the present invention includes an electrical discharge machining system including a fixture for the post which allows the post to be mounted and clamped thereto in a precise predetermined position thereon whether the post has a round cross-sectional configuration or a polygonal shape thereto. In a preferred form, the fixture includes a bore for a round shaped post having an indexing pin which seats in a groove formed in an upper surface of the fixture and is clamped therein by clamp members actuated by cam clamp operators. The clamping of the pin in the groove keeps the post rigid and stationary relative to the fixture to provide for improved results when the electrode is subject to machining operations. In addition, because the clamping mechanism herein does away with the previously-described hydraulic clamping device utilized with prior fixtures, there are no hydraulic lines or connections that have to be set up and maintained to provide an easier-to-use fixture for use with the tool holding system of the present invention.
In one form of the invention, a tool holding system is provided including a tool for removing metal material from a workpiece and having a rear mounting surface with an aperture formed therein. A tool holder is provided for mounting the tool in predetermined positions thereon and includes an elongate post having a longitudinal axis and a first end at which the tool is mounted and a second end opposite the first end along the axis. The post includes a longitudinally extending side surface parallel to the axis having one of: (1) a curved portion, and (2) at least two angled portions with their angles being at other than a right angle. The post has a tool mounting surface of the post at the first end thereof and with an aperture formed therein. The mounting surfaces include interfitting surface portions which cooperate to define the predetermined positions of the tool on the post relative to X-Y directions transverse to the post axis and a C-direction angularly thereabout with the apertures of the mounting surfaces being in alignment with each other. A fastener can be received in the aligned apertures to clamp the interfitting surface portions together with the tool located in one of its predetermined positions on the post for fixing the tool against shifting relative to the post during machining operations.
In a preferred form, the tool is an electrode for removing metal from a workpiece by electric discharge machining, and at least one of the electrode and post includes a throughbore to provide a removable attachment between the electrode and post with the fastener seated in the throughbore, and the interfitting surface portions cooperating to provide accurate and repeatable attachment of different electrodes onto the post. In this manner, the present invention provides machinists with the ability to obtain precisely formed and dimensioned electrodes, and further allows these electrodes to be used on a single post without having to dispose of posts once the electrode has been worn through use as with prior systems that utilize glue to hold the electrode to the post.
The interfitting surface portions can include mating raised and recessed portions extending in a generally cross-shaped pattern on the mounting surfaces with the apertures being generally centrally located thereon relative to the cross-shaped surface portions of the respective post and tool and aligned along the post longitudinal axis when the tool is fastened and clamped to the post in one of its predetermined positions thereon. Preferably, the mating raised and recessed portions are in the form of prismatic shaped raised and recessed portions of the tool and post.
Where the post side surface includes the at least two portions having their angles at other than a right angle, the two portions have opposite inclined flat sections and opposite parallel flat sections with the inclined flat and parallel sections of each portion meeting and defining an obtuse angle therebetween. A fixture having an predetermined generally v-shape is provided for the post described above for receiving the inclined flat sections thereof flush thereon with the parallel flat sections extending in a generally vertical direction. In this manner, the machinist has flat vertically oriented faces on the post for reference and from which they can obtain the dimensions of the electrode in a relatively easy and standard manner.
In another form where the post side surface includes the curved portion, the curved portion is provided with a round shape and the post has an indexing pin extending therefrom normal to the post axis. For this round shaped post, one fixture that can be used is a v-fixture having an upper surface with a predetermined generally v-shape, and a vertical surface having a vertical recessed groove extending downward centrally from the v-shaped upper surface with the groove receiving the indexing pin therein so as to provide accurate, repeatable positioning of the round post when clamped in the fixture.
In one form, the aperture of the tool includes an enlarged countersunk portion to allow for front mounting of the tool to the post with the fastener seated in the enlarged portion of the tool aperture. In this manner, when it is desired to replace the tool on the post such as when an electrode is consumed, an operator can readily access the fastener through the tool without having to release the post from its fixture in the machine. Thereafter, the next tool can be readily attached to the post through the front mounting provided by the tool.
In another form of the invention an electrical discharge machining system is provided including an electrode having a predetermined, precisely formed shaped and an electrode mounting post having a generally round cross-sectional configuration and a longitudinal axis extending therethrough. The electrode and post include mounting surfaces having mating portions which define a plurality of predetermined positions for mounting of the electrode to the post, and apertures in the mounting surfaces which are aligned with each other with the electrode located in one of its predetermined positions for receiving a fastener to clamp the mating portions together. The post includes an indexing pin extending normal to the longitudinal axis thereof. A fixture for the post is provided and includes a surface having a bore of round cross-sectional shape therein for receiving the round post. Grooves are recessed in the fixture surface and extend generally radially from the bore for receiving the indexing pin therein with the post in the bore. Clamp members are spaced from the groove for being shifted toward the grooves to clamp the pin in one of the grooves. Cam clamp operators are mounted to the fixture and are operable to shift the clamp members toward the grooves for clamping of the pin in the groove to locate the post angularly relative to the fixture for accurate machining of the electrode to form the precise predetermined shape thereof and of workpieces with the precisely formed electrode mounted to the posts.
The EDM system described-above is advantageous in providing positive locating of the electrode mounting posts relative to the fixture by way of the grooves. Further, the fixture provides for improved holding power due to the clamping action provided by one of the clamp members on the pin in the groove which is particularly important during machining operations on the electrode as with prior hydraulic clamping there was a tendency for the posts to shift when exposed to high cutting forces on the electrode which limited the ability to obtain the precision necessary for forming the desired precise shaped thereon.
Another aspect of the invention is a method for machining with a reusable electrode holder. The method includes providing electrodes and a mounting post therefor with mounting surfaces having interfitting surface portions to provide predetermined mounting positions of the electrodes on the post, seating one electrode on the mounting post in one of the predetermined positions with the interfitting surface portions in mating relation to each other, clamping the interfitting surface together with a fastener received in aligned apertures of the electrode and post, performing one of a standard metal cutting machining operation on the one electrode for forming precision electrodes and an electric discharge machining operation on a workpiece with the one precision electrode fastened and clamped to the post, releasing one of the precisely formed electrode, and the one used and worn electrode from the post by removal of the fastener, replacing the one electrode with a different electrode on the post with a second electrode in one of the predetermined positions on the post in an accurate and repeatable manner due to the provision of the interfitting surface portions for one of precise machining of the second electrode, and electric discharge machining with the precision formed second electrode.